The invention relates to a steering boost arrangement for motor vehicles with a steering shaft with coaxially disposed electric rotor disc motor and a reducing gear acting onto the steering shaft.
EP 0 124 790 discloses a steering boost with electric drive which is coaxially installed in the steering shaft. The servo force coupling-in takes place with the aid of a servo motor which acts onto the steering shaft via a planetary gear. The gear and the motor are disposed coaxially relative to the steering shaft, with the steering shaft being guided centrally through the arrangement. Since the motor and the gear are developed as a separate assembled unit, the arrangement becomes voluminous and comprises a large number of parts, which is disadvantageous with respect to economy and reliability. A special disadvantage comprises that the sun wheel of the planetary gear is fastened on the rotor of the motor drive with the rotor and the sun wheel being rotatably disposed and supported coaxially with the shaft axis. This makes clear that in this embodiment the sun wheel always has a relatively large diameter, but at least can never have a smaller diameter than the steering shaft itself. Since the reduction ratio of a planetary gear is determined in the first step by the ratio of the diameters of the sun wheel and the planet wheels, it can readily be seen that in this embodiment large diameter ratios of the wheels cannot be attained since the sun wheel itself in this construction always has a relatively large diameter. In the description of the previously described application, a reduction ratio of 1:1 to 1:10 is specified. Larger reduction ratios would only be possible in the present embodiment if the entire gearing diameter were drastically increased, which would lead to impermissible dimensions of the steering boost arrangement.
In addition, small gear reduction ratios have the disadvantage that the motor power must be correspondingly large, and furthermore only slowly rotating motors, which require corresponding expenditures, can be employed. A further disadvantage herein is the less rapid response behavior which leads to reaction inertias in the system.
A further known embodiment comprises employing worm gearing configurations as the reduction gear. But due to the poor overall efficiency, high driving power is required, which, in turn, lead to large overall constructional forms of the servo unit and limit an economic implementation. The rotational axes offset by 90xc2x0 in the worm gearing impede a compact arrangement.
Due to the voluminous construction also less structural space is available primarily in the direction of the shaft axis, which is of disadvantage primarily with respect to the safety functions. In the event of an impact of the driver onto the steering wheel, the energy must be absorbed as advantageously as possible in order for the motor vehicle driver not to be injured. This is attained thereby that in such case the steering spindle is to yield such that it absorbs the energy by telescopingly sliding one into the other. This forms the basis for the requirement of being able to provide sliding paths of maximum length for the energy absorption device, which is not readily possible with the known servo drives according to prior art. The required large installation space in the axial direction thus is at the expense of the desired energy absorption sliding path in the event of impact.
A further disadvantage in known devices comprises that, due to the complicated structuring, apart from mechanical play, also due to the large masses to be moved, these have a certain reaction inertia, which has a negative effect onto the steering behavior and the economy. The reaction inertia leads, for example, to greater response times of the steering, which, for example during rapid evasion maneuvers, is of disadvantage. Complex constructions, corresponding to the known arrangements, disadvantageously consume much energy during operation. Apart from the expenditures for the energy provision of the necessary energy, additional expenditures are necessary in order to dissipate the corresponding heat losses.
A further disadvantage of known arrangements comprises that the compensation of plays in driving parts is complicated and expensive and is only conditionally possible. This leads to less precise steering behavior and to increased reaction times, connected with a less favorably perceived steering sensation of the motor vehicle driver.
The task of the present invention lies in eliminating the disadvantages of the previously described prior art. The task lies in particular in realizing an electric steering boost for a steering arrangement which is built highly compactly and which permits attaining rapid response behavior with good dynamics with responsive steering behavior for the driver, and which operates at high efficiency and can be produced economically.
The task is solved according to the invention by the arrangement according to the features of claim 1. The dependent claims define further advantageous embodiments.
The task is solved according to the invention thereby that a disc-shaped electric motor is combined with a spur gearing with the motor being installed coaxially in the shaft of a steering spindle, a steering shaft or a steering gear pin, and the spur gearing is disposed eccentrically with respect to the shaft and coupled to it.
The rotor, which is disposed rotatably and supported about the shaft, drives a tubular fitting-shaped toothed wheel, also coaxially rotating about the shaft, which is developed as a pinion. This pinion, together with an eccentrically disposed spur wheel of greater diameter, which engages it, forms a first stage of a spur reduction gearing. This spur wheel is provided equirotatingly with a further toothed wheel, which acts onto a toothed ring of a further toothed wheel, which is connected with the steering shaft and thus transmits the motor force or acts onto the steering shaft reduced in terms of the rotational number. The sheering shaft is not interrupted and leads with its axis through the arrangement. The gear arrangement can be developed to be single-stage or multi-stage. The necessary gear reduction is defined in known manner through the ratios of the engaged numbers of teeth of the toothed wheel pairs or through its diameter ratios and through the stage number of the gear arrangement.
The stator of the rotor disc motor as well as the gear arrangement are fixed on a carrier plate, or the toothed wheels are rotationally movably supported. The carrier plate is disposed substantially transversely to the steering shaft axis and parallel to the rotor disc plane, and the carrier plate incorporates also housing parts which cover and protect the motor and the gear arrangement. The carrier plate or the housing covers disposed on it comprise fastening means, for example a fastening flange in order to hold the carrier plate with the motor stator and gear elements stationarily on the motor vehicle chassis, preferably with interspaced rubber bearings. The steering shaft itself, which is guided through the arrangement, is preferably supported bilaterially in the proximity of the entrance and exit with respect to housing parts or the carrier plate. As bearings serve for this purpose preferably friction-free rolling bearings such as ball bearings.
As the motor driving are preferably employed electronically commutating rotor disc motors, which comprise a stator winding disposed stationarily, wherein between the stator windings a rotating disc is provided as the rotor, which comprises flat permanent magnets. In order to be able to generate high moments, strong permanent magnets are preferably employed, which have magnetic energy capacity of more than 6000 Gauss, such as, for example, magnetic materials comprising rare earths, such as for example cobalt samarium material and, in particular, neodymium magnetic materials. The motor is controlled via power electronics with regulation, which measures via a torque measurement on the steering wheel side of the steering shaft the steering torque coupled-in by the motor vehicle driver and via the regulator controls the motor-gear unit such that the steering force is correspondingly augmented. The torque measurement takes place at the steering wheel side before the servo unit, for example with wire resistance strain gauges (DMS) or preferably via the deformation measurement of a torsion rod installed in the steering shaft.
The servo arrangement should be capable of outputting torques to the shaft which are  greater than 50 Nm, preferably up to 100 Nm. The driver should herein not have to apply more than maximally 10 Nm, wherein preferably no more than 6 Nm are desired in order to make possible steering with easy action. Furthermore, the torque sensor should already respond at a rotation of  less than 1xc2x0, preferably  less than 0.5xc2x0 in order to ensure precise steering. Said torque values can be attained with the advantageous inventive combination of a rotor disc motor with the corresponding spur gearing. Through the degrees of freedom already described in the dimensioning of the motor and the gear dimensioning, the corresponding torques can be attained via the number of stages and reduction ratios per stage. Herein especially the compact construction of the arrangement are to be taken into consideration and, in addition, the arrangement should not require too much electric supply power. High motor power would increase the structural size as well as also the cooling expenditures and necessitate a greater expenditure in the control electronics. Moreover, this would require stronger light machines, greater batteries and thicker current lines. The economy would in this case no longer be given. With the arrangement according to the invention, torques of at least 50 Nm, even more than 65 Nm, can be readily achieved at maximum supply motor currents of 75 A and a voltage of 12 V. At the corresponding total optimization of the motor-gear arrangement with respect to efficiency up to 75 Nm can be attained at said motor supply. Dimensions of the motor-gear-servo arrangement are herein possible in the range of maximally 100 mm in thickness, maximally 300 mm in height and maximally 200 mm in width. With a typical embodiment example in which a two-stage gear, such as shown in the Figures, has been employed, a maximum thickness of 84 mm could be attained at a height of maximally 246 mm and a width of maximally 173 mm. The two-stage arrangement corresponding to this example is therefore an especially suitable embodiment for the present application. In this embodiment example an overall efficiency of motor and gearing of more than 80% was attained at a torque transfer of 80 Nm.
In the present embodiment overall motor-gear efficiency of better than 75% can readily be achieved. It is even possible to attain values above 85%, which is especially favorable for economic operation. In order to optimize the quietness of running and the play of the toothed wheel, it is furthermore proposed to implement the toothed wheels in engagement such that they are obliquely denticulated. Especially high level quietness of running is attained if all engaged toothed wheel pairs are implemented to be obliquely denticulated. The present structuring with the spur gearing, in addition, makes possible that means are provided which optimally permits setting the toothed wheel play. In order to obtain, as stated, good steering behavior, a servo drive with maximum freedom from play is important. But this can be in contradiction to a high gear efficiency, if the freedom from play leads to increased friction values. The setting of the toothed wheel play thus permits, on the one hand, minimizing the toothed wheel play and, on the other hand, maximizing the gear efficiency. At the values stated and with the two-stage implementation, reduction ratios of 20 to 30 have proven to be especially good for the gear arrangement.
The entire arrangement is operated upon actuation of the steering shaft via sensor elements, evaluation electronics and power electronics such that, corresponding to the actuation of the steering shaft by the driver, a force coupling onto the steering shaft takes place and thus easy-running steering is possible. The arrangement leads to an extremely compact construction which permits high efficiency and, in addition, can be well cooled at low expenditures. The required ergonomic steering behavior can be realized and, through the compact embodiment, especially with the low axial structural length, the absorption elements necessary for the event of a crash, can be optimally realized and installed.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure and are entirely based on the Swiss priority application No. 600/99.